(1) Field of the Invention
The present invention relates to a control signal generation circuit for generating a control signal needed to correctly record information on an optical disc using light of, for example, a laser, or correctly play back the information recorded on an optical disc, a control device for performing an optical control using a control signal, and an optical disc apparatus that is equipped with the control signal generation circuit and the control device, and records and plays back the information on an optical disc.
(2) Description of the Related Art
An optical disc apparatus for performing a tracking control has been conventionally proposed. (For example, refer to Japanese Laid-Open Patent application No. 5-151592.)
FIG. 1 is a block diagram showing the structure of the above-mentioned conventional optical disc apparatus.
The above-mentioned conventional optical disc apparatus adjusts gain balance according to the symmetry of a tracking error signal which will be mentioned later, and adjusts the lens position so as to reduce jitters to a minimum. The optical disc apparatus comprises an optical pick up 10, a control signal generation circuit 20, LPF 22 and 24, both of which consist of a lowpass filter, a digital signal processor 40 (called “DSP” from here), and a driving circuit 2.
The optical pick up 10 irradiates a converged optical beam 11 on a track of the information recording surface of an optical disc 1 and receives the reflection light. The optical pick up 10 comprises a laser radiation element (not shown as a figure) for outputting the optical beam 11, a convergence lens 12 for converging the optical beam 11, an actuator 13 for shifting the convergence lens 12 in the tracking direction, and a light detection device 14 for receiving and detecting the reflection light. The “tracking direction” used here means the direction for traversing the tracks on the information recording surface of the optical disc 1, that is, the diameter direction of the optical disc 1.
Also, the light receiving area of the light detection device 14 is divided into two in the tracking direction. A detection unit 14a corresponding to one of the areas divided into two detects inside of the reflection light (the inner radius of the optical disc 1), and a detection unit 14b corresponding to the other area detects outside of the reflection light (the outer radius of the optical disc 1). After that, the detection units 14a and 14b output the detected result respectively as a detection signal to the control signal generation circuit 20.
The control signal generation circuit 20 outputs a tracking error signal TE and an addition signal AS1 by performing a signal processing on the detected signals from the detection units 14a and 14b, and comprises a gain balance circuit 30, a subtraction circuit 21 and addition circuit 23.
The gain balance circuit 30 consists of a gain circuit 30a for amplifying the detection signal outputted from the detection unit 14a and a gain circuit 30b for amplifying the detection signal outputted from the detection unit 14b. Also, the gain balance circuit 30 increases or decreases respective gains of the gain circuits 30a and 30b independently based on the control from DSP40, and changes the gain balance of the detection signals from the detection units 14a and 14b. The gain balance used here indicates the ratio of the gain in the gain circuit 30a to the gain in the gain circuit 30b. 
The subtraction circuit 21 calculates the output difference between the gain circuits 30a and 30b, and outputs the result as a tracking error signal TE.
The addition circuit 23 calculates the addition result of the detected signals from the detection units 14a and 14b, and outputs the result as an addition signal AS1.
The DSP 40 adjusts the gain balance of the gain balance circuit 30 of the control signal generation circuit 20 based on the tracking error signal TE outputted via LPF22 from the control signal generation circuit 20 and the addition signal AS1 outputted via LPF24 from the control signal generation circuit 20, adjusts the lens position of the convergence lens 12 of the optical pick up 10, and performs a tracking control.
This DSP 40 comprises A/D converters 41 and 51, an auto gain control unit 52 (called “AGC” from here), an offset adjustment unit 42, a gain adjustment unit 43, a tracking control unit 44, a symmetry detection unit 61, a balance adjustment unit 62, an amplitude detection unit 71, a lens position adjustment unit 72, a lens position setting unit 45 and a D/A converter 46.
The A/D converter 41 converts the tracking error signal TE from analog to digital and outputs the signal to the offset adjustment unit 42, while the A/D converter 51 converts the addition signal AS1 from analog to digital and outputs the signal to the AGC unit 52.
The offset adjustment unit 42 detects the offset factor which occurs in the circuit to the tracking error signal TE under the condition where no spot of reflection light is found by the light detection device 14, for example, when the laser radiation element is off or the focus of the convergence lens 12 is taken away from the information recording surface of the optical disc 1. The offset adjustment unit 42 also adds an offset appropriate to the factor to the tracking error signal TE, and outputs the addition result.
The AGC unit 52 specifies the gain of the tracking error signal TE according to the addition signal AS1 outputted from the A/D converter 51 to the gain adjustment unit 43.
The gain adjustment unit 43 adjusts the gain of the output from the offset adjustment unit 42 (the tracking error signal TE) according to the above-mentioned instruction from the AGC unit 52.
The tracking control unit 44, on obtaining the tracking error signal TE outputted from the gain adjustment unit 43, calculates the tracking driving value by filter operation for performing phase compensation or low frequency compensation, and outputs the tracking control signal showing the tracking driving value to the lens position setting unit 45.
The amplitude detection unit 71, on obtaining the tracking error signal TE outputted from the offset adjustment unit 42, detects the amplitude and outputs the detected result to the lens position adjustment unit 72.
The lens position adjustment unit 72 specifies the optimum lens position of the convergence lens 12 so that the reflection light that passes through the convergence lens 12 can be received by the detection units 14a and 14b evenly in space, and outputs, to the lens position setting unit 45, a lens position adjustment signal for adjusting the lens position of the convergence lens 12 to the determined lens position.
The lens position setting unit 45 adds the tracking control signal from the tracking control unit 44 to the lens position adjustment signal from the lens position adjustment unit 72 and outputs the addition result as the tracking driving signal to the D/A converter 46.
The D/A converter 46 converts the tracking driving signal from the lens position setting unit 45 from digital to analog and then outputs the signal to the driving circuit 2.
The symmetry detection unit 61, on obtaining the tracking error signal TE from the offset adjustment unit 42, detects the symmetry of the tracking error signal TE and outputs the result to the balance adjustment unit 62.
The balance adjustment unit 62, on obtaining the detected result of the symmetry detection unit 61, changes the gain balance of the gain balance circuit 30 to the optimum gain balance so as to equalizes the outputs from the gain circuits 30a and 30b when the detection units 14a and 14b receive the same amount of light based on the detected result.
The driving circuit 2, on obtaining the tracking driving signal outputted from the DSP 40, drives the actuator 13 of the optical pick up 10 by amplifying the current of the tracking driving signal and outputting the signal.
In this way, the convergence lens 12 of the optical pick up 10 is shifted in the tracking direction in a way that the spot of the optical beam 11 can follow the tracks of the optical disc 1 making the lens position determined by the lens position adjustment unit 72 the center.
The conventional optical disc apparatus like this adjusts the lens position of the convergence lens 12 to the optimum lens position (lens position adjustment) first, and adjusts the gain of the balance circuit 30 in the control signal generation circuit 20 to the above-mentioned optimum gain balance (gain balance adjustment). After the lens position adjustment and the gain balance adjustment are performed, the optical disc apparatus plays back the information recorded on the information recording surface of the optical disc 1 performing a tracking control.
Here, the above-mentioned lens position adjustment and gain balance adjustment will be explained in detail respectively.
As the convergence lens 12 deviates from the center position (the above-mentioned optimum lens position) of the light detection device 14 in the initial state, the spot of the reflection light forms an image deviating from the light detection device 14. A deviation of the lens position like this may occur because of the inclination of the optical axis of a lens triggered by the setting error of optical parts inside the optical disc apparatus or because of the self weight of the convergence lens 12 depending on the setting condition of the optical disc apparatus. For example, when the optical disc apparatus is vertically set (the optical disc apparatus is set in a way that the convergence lens 12 is set vertically), the convergence lens 12 hangs over in the vertical direction because of its self weight, and the convergence lens 12 shifts far away from the center position in the initial state.
Therefore, as to the lens position adjustment, the lens position of the convergence lens 12 is adjusted so as to equalize the spot sizes of reflection light received by the detection units 14a and 14b of the optical pick up 10.
More specifically, the lens position adjustment unit 72 of the DSP 40 shifts the convergence lens 12 in the tracking direction within the predetermined range, and obtains the results at the respective lens positions detected by the amplitude detection unit 71. The amplitude detection unit 71 of the DSP 40 calculates the difference (TEmax-TEmin) between the maximum level (TEmax) and the minimum level (TEmin) of the tracking error signal TE outputted from the offset adjustment unit 42 at the respective lens positions of the convergence lens 12, obtains the TE amplitude, and outputs this as the detected result to the lens position adjustment unit 72.
FIG. 2 is an illustration showing the tracking error signal TE and the TE amplitude when the convergence lens 12 is set at the predetermined lens position.
As shown in this FIG. 2, the amplitude detection unit 71 obtains the tracking error signal TE that repeatedly fluctuates as time passes, and obtains the TE amplitude from the difference between the maximum level (TEmax) and the minimum level (TEmin).
And, the lens position adjustment unit 72 specifies the lens position where the TE amplitude becomes maximum as the optimum position.
FIG. 3 is a diagram showing the relation between the lens position of the convergence lens 12 and the TE amplitude.
As shown in this FIG. 3, the TE amplitude changes depending on the lens position of the convergence lens 12, and the TE amplitude is the maximum value at the predetermined lens position.
The lens position adjustment unit 72 stores TE amplitudes at respective lens positions, for example, A, B, C, D, and E which are points shown in FIG. 3, specifies the lens position where the TE amplitude is maximum (the lens position shown as point D) as the above-mentioned optimum lens position, and outputs a lens position adjustment signal that makes it possible to shift the convergence lens 12 to the lens position.
Consequently, the driving circuit 2 that obtains the lens position adjustment signal like this through the lens position setting unit 45 and the D/A converter 46 drives the actuator 13 based on the lens position adjustment signal and shifts the convergence lens 12 to the determined lens position. In this way, the lens position adjustment is performed.
Next, the gain balance adjustment will be explained.
The detection sensitivities of detection units 14a and 14b of the light detection device 14 are rarely equal to each other for manufacturing reasons, in other words, they are usually different from each other. Therefore, when the gain balance adjustment is not performed, in other words, gains of the gain circuits 30a and 30b are made to be equal to each other, an offset is needed for the tracking error signal TE because of the output difference between the detection units 14a and 14b. 
Therefore, the symmetry detection unit 61 and the balance adjustment unit 62 of the DSP 40 perform the gain balance adjustment so that the offset factor for the above-mentioned tracking error signal TE can be removed.
More specifically, the symmetry detection unit 61 adds the maximum level of the tracking error signal TE (TEmax) to the minimum level of the tracking error signal TE (TEmin), and outputs the addition result (TEmax+TEmin) to the balance adjustment unit 62. And, the balance adjustment unit 62 changes the gains of the gain circuits 30a and 30b of the gain balance circuit 30 so that the addition result by the symmetry detection unit 61 becomes “0”, in other words, the tracking error signal TE has a wave form symmetrical in the positive and negative directions, and adjusts the gain balance of the gain balance circuit 30. In this way, the gain balance adjustment is performed.
A series of operations of the above-mentioned conventional optical disc apparatus like this will be explained with reference to FIG. 4.
FIG. 4 is a flow chart showing the operation of the above-mentioned conventional optical disc apparatus.
First, the lens position adjustment unit 72 drives the actuator 13 by controlling the driving circuit 2 and sets the lens position of the convergence lens 12 at x1 (step S900).
Next, the amplitude detection unit 71 obtains the tracking error signal TE when the convergence lens 12 is set on the lens position x1 (step S902) and detects the TE amplitude w1 (step S904).
The lens position adjustment unit 72 sets lens positions of the convergence lens 12 at x2, x3, . . . xn in order by controlling the driving circuit 2 and detects the TE amplitudes w2, w3, . . . wn (steps S900˜S906) by repeatedly executing the operations of the above-mentioned steps from S902 to S904 on the respective lens positions x2, x3, . . . xn.
Next, the lens position adjustment unit 72 specifies, for example, the lens position x5 corresponding to the TE amplitude w5 with the maximum TE amplitude out of the detected TE amplitudes w1, w2, w3, . . . wn as the optimum lens position (step S908) and outputs the lens position adjustment signal that makes the lens position of the convergence lens 12 x5. In this way, the driving circuit 2 drives the actuator 13 based on the lens position adjustment signal and adjusts the lens position of the convergence lens 12 to the determined lens position x5 (step S910).
A lens position adjustment is performed by the operation in the steps from S900 to S910.
The symmetry detection unit 61 and the balance adjustment unit 62 obtains the tracking error signal TE (step S912), detects the symmetry of the tracking error signal TE (step S914) and sets the gain balance of the gain balance circuit 30 at the optimum gain balance in a way that the difference between the maximum level of the tracking error signal TE (TEmax) and the minimum level of the tracking error signal TE (TEmin) becomes “0” (step S916).
The gain balance adjustment is performed by the operation in the steps from S912 to S916.
Next, when information reading or writing is performed on the optical disc 1, the tracking control unit 44 obtains the tracking error signal TE via the gain adjustment unit 43 and outputs the tracking control signal corresponding to the tracking error signal TE. In this way, the optical disc apparatus performs a tracking control that makes the beam spot of the optical beam 11 follow the tracks of the optical disc 1 (step S918).
In this way, the gain balance of the above-mentioned conventional optical disc apparatus needs to be pre-adjusted after the lens position adjustment is performed.
However, the gain balance of the above-mentioned conventional optical disc apparatus is not adjusted when the lens position adjustment is performed. Therefore, there is a case where the TE amplitude becomes big when the convergence lens 12 shifts to either of the detection unit 14a or 14b of the light detection device 14 with a bigger detection sensitivity, consequently, it becomes impossible to accurately perform a lens position adjustment. Also, even when the detection sensitivities of the respective detection units 14a and 14b are equal to each other at the time of performing a lens position adjustment, many errors are included in the TE amplitudes detected by finding the peak from the tracking error signal TE shown in FIG. 2, also, many errors are included in the lens positions specified from the relations between the TE amplitudes and lens positions shown in FIG. 3, thus it is impossible to accurately perform a lens position adjustment.
The above-mentioned conventional optical disc apparatus cannot accurately perform a lens position adjustment because it performs a tracking control under the condition where the lens position deviates from a right position, which brings a problem that it is impossible to realize a stable tracking control because of the lens position deviation.
Also, the above-mentioned conventional optical disc apparatus cannot accurately perform a gain balance adjustment either when it cannot accurately perform a lens position adjustment mentioned above because it adjusts the gain balance under the condition where the lens position is adjusted.
In other words, the above-mentioned conventional optical disc apparatus performs a tracking control under the condition where the gain balance deviates because it cannot accurately perform the gain balance adjustment, which brings a problem that it cannot realize a stable tracking control because of the gain balance deviation.
Further, in the above-mentioned conventional optical disc apparatus, the addition signal AS1 is the addition result of the detected signals outputted from the detection units 14a and 14b. As the addition signal AS1 changes as the lens position of the convergence lens 12 shifts depending on the difference of sensitivities between the detection units 14a and 14b and the gain of the tracking error signal TE is changed according to the addition signal AS1, performing a tracking control that shifts the lens position further causes a problem of making the tracking control more unstable.